LIPOPHILIC FLAVONOIDS FROM ORTHOSIPHON-SPICATUS AS INHIBITORS OF 15-LIPOXYGENASE

Lipophilic, O-methylated flavonoids from the East Asian medicinal plant Orthosiphon spicatus have been tested as inhibitors of 15-lipoxygenase from soybeans, with linoleic acid and arachidonic acid as substrates. Sinensetin (5,6,7,3',4'-pentamethoxyflavone) and tetramethylscutellarein (5,6,7,4'-tetramethoxyflavone) are the strongest inhibitors, with IC50 values of 114 +/- 5 and 110 +/- 3 muM towards oxidation of linoleic acid. Quercetin, a well-known flavonoid inhibitor of lipoxygenases which was tested for comparison, had an IC50 value of 98 +/- 3 muM in this system. Other Orthosiphon flavonoids closely related to sinensetin and tetramethylscutellarein were weaker inhibitors or inactive. At low concentrations, sinensetin and tetramethylscutellarein both showed uncompetitive inhibition. Inhibition of arachidonic acid oxidation catalyzed by 15-lipoxygenase did not differ appreciably from that of linoleic acid. HPLC analysis of flavonoids from O. spicatus gave concentrations of sinensetin and tetramethylscutellarein of 0.19% and 0.11%, respectively, in dried plant tissue, while concentrations of the other flavonoids were lower. An ethyl acetate extract of the plant inhibited 15-lipoxygenase catalyzed oxidation of linoleic acid with an IC50 value of 0.018 +/- 0.002% (w/v). A synergistic effect of the flavonoids was observed, but considerably less than was needed to account for the activity of the extract. Thus, the existence of other, as yet unidentified, 15-lipoxygenase inhibitors in Orthosiphon spicatus cannot be excluded.